Scientists call organisms that can survive under extreme conditions -- from a human perspective -- "extremophiles." Extremophiles can tolerate, and even thrive in conditions that would be lethal to humans. This can include extreme cold, heat, salinity or pH. Many simple organisms can survive at such extremes. Even some higher organisms can survive pH extremes.

Extremophiles and pH

The word "extremophile" comes from the Latin word "extremus," or extreme, and the Greek root "-phile," meaning "-loving." Several types of extremophiles exist, including organisms that can tolerate very low or very high pH ranges. Scientists use pH to describe the chemistry of a given sample, specifically the concentration of hydrogen and hydroxide ions. The scale goes from 0 to 14, with 7 at the neutral point. Samples with pHs lower than 7 are acidic, and ones with pHs higher than 7 are basic or alkaline. The pH scale is logarithmic, which means that each integer represents a tenfold change. So a sample with a pH of 2 is ten times more acidic than a sample with a pH of 3. Under very unusual circumstances, a sample's pH can go lower than 0 and higher than 14. Organisms that prefer an extremely acidic pH are called acidophiles and organisms that prefer very basic pHs are called alkaliphiles.

Record Holders

Most of the record holders belong to a vast group of organisms called prokaryotes. Prokaryotes are relatively simple, single-celled organisms without membrane-bound organelles. Bacteria and Archeans are prokaryotes. Both of the record-holders for pH are bacteria. "Raven and Johnson's Biology," sixth edition, provides information about these record holders. The acidophile that can tolerate the lowest pH is a bacterium called Picrophilus oshimae It thrives at a pH of 0.7, but can survive a pH of -0.06. For reference, battery acid has a pH of 0.0. Interestingly, P. oshimae is also a moderate thermophile, and thrives at temperatures around 60 degrees Celsius, or 140 degrees Fahrenheit. It lives in hot springs. The organism that survives at the highest pH is Natronobacterium magadii, a bacterium that thrives at a pH of 10, and can survive at up to 12, which is roughly the pH of household bleachs. It is also an extreme halophile or salt-loving extremophile, preferring water that is 20% salt. However, in the category of salt tolerance, a bacterium called Halobacterium salinarum beats it at supersaturated 32% salt.

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Eukaryotes, Living on the Edge

Eukaryotes have larger and more complex cells than bacteria. Humans fit into this category. However, most of the record-holders in this group are one-celled eukaryotes called protists. Dave Roberts from the Natural History Museum writes about these extreme eukaryotes. Some acidophile eukaryotes approach bacteria's record. A protist called Cyanidium caldarium and three simple fungi can tolerate a pH near 0. On the alkaline side of the coin, the record-holding eukaryotes can survive at a pH of around 10 in several soda lakes in Africa. This is about the pH of antacid tablets. These lakes feature about twenty species of native protists and even three species of rotifer, tiny multicellular animals that thrive under these conditions.

Vertebrates: Not Even Close

Vertebrates are eukaroyotic animals with a backbone. For the most parts, vertebrates cannot hold a candle to simpler organisms when it comes to tolerating extremes. The lowest pH a fish can survive is around 3.1, a record held by the cardinal tetra (Cheirodon axelrodi), according to the study "Exceptional tolerance to low pH of some tropical blackwater fish," published in the Journal of Experimental Zoology. The study goes on to say, however, that cardinal fish can only tolerate this extreme pH for a few weeks, and need a higher pH of at least 3.5 to thrive. This isn't the limit, apparently, for vertebrates, however. According to a 2012 Portland State Vanguard article, the eggs of some species of killifish can survive lower pH values, around 2.0, but only because the eggs form a hard, glass-like coating to protect the embryo until better conditions arise. For reference, a pH of 2.0 to 3.0 is a range that includes everyday substances like lemon juice and vinegar. Here is one last example. According to a study published in the Journal of Pineal Research, an African fish called Oreochromis alcalicus grahami can survive in water with a pH up to 10.0, but develops signs of stress.